The present invention relates to an ethylenic copolymer, an ethylenic copolymer composition, and an ethylenic copolymer film. More precisely, it relates to an ethylenic copolymer which has a narrow molecular weight distribution and a narrow branching degree distribution and has a controlled compositional distribution profile and which can be formed into a film having good mechanical characteristics including tear strength, etc., good heat-sealing properties and good ESCR (environmental stress crack resistance); to an ethylenic copolymer composition containing said copolymer; and to an ethylenic copolymer film having excellent characteristics such as those mentioned above.
Heretofore, ethylenic copolymers such as ethylene-xcex1-olefin copolymers have been being used widely in various fields as popular resins. Ethylenic copolymers are produced in the presence of heterogeneous catalysts. However, ethylenic copolymers as produced in the presence of heterogeneous catalysts have a broad molecular weight distribution and a broad branching degree distribution, and contain a large amount of low-molecular components having a high branching degree. Therefore, it is known that films to be formed from such ethylenic copolymers have low impact strength, poor heat-sealing properties and low ESCR. On the other hand, as having a low degree of anisotropy (imbalance between the lengthwise strength and the widthwise strength), the films are advantageous in that they have high tear strength which is important as one physical property of films.
Recently, homogeneous metallocene catalysts having a high catalytic activity and capable of catalyzing high copolymerization have been developed. Ethylenic copolymers as produced in the presence of such metallocene catalysts shall have a narrow molecular weight distribution and a narrow branching degree distribution (that is, the distribution of the number of branches constituting the copolymer is narrow), and contain a small amount of low-molecular components having a high branching degree. Therefore, it is known that films of such ethylenic copolymers can have improved heat-sealing properties, impact strength and ESCR.
Some ethylenic copolymers as produced in the presence of such metallocene catalysts (often referred to as single-site catalysts) are known. For example, U.S. Pat. No. 5,272,236 discloses ethylene-xcex1-olefin copolymers as produced in the presence of metallocene catalysts. Since the copolymers disclosed have a narrow compositional distribution, films of the copolymers may have improved impact strength and ESCR. However, since the films have a large degree of anisotropy, they are defective in that they have low tear strength. On the other hand, single-site, linear low-density polyethylenes are commercially available. For example, known is a commercial product of Exact which is a trade name of Exxon. However, since this has a narrow compositional distribution and comprises a comonomer of hexene-1 or an xcex1-olefin lower than hexene-1, the tear strength of films to be formed from it is not improved to such a degree as expected, though the heat-sealing property of the films may be desirably improved.
Given this situation, various attempts have heretofore been made to blend different copolymers thereby to control the branching degree distribution and the molecular weight distribution of the resulting copolymer mixtures. However, regarding the tear strength and the impact strength of films to be formed from such copolymer mixtures, if one of the two is increased, the other is lowered. At present, therefore, films having both high tear strength and high impact strength are unknown.
Japanese Patent Application PCT-through Laid-Open No. 502710/1991 discloses a composition comprising an ethylenic copolymer as obtained in the presence of a homogeneous catalyst. However, this refers to nothing about the impact strength of films of the composition, though referring to the improvement in the tear strength of the films. Japanese Patent Application Laid-Open Nos. 136195/1994, 136196/1994 and 207057/1994 disclose compositions comprising two different copolymers both having a narrow compositional distribution and characterized by their melt tension but each having a different density and a different melt flow rate (MFR), and the compositions disclosed are said to form films with improved impact strength. However, the compositions disclosed are characterized by comprising copolymers each having a different molecular weight, and are therefore defective in that their molecular weight distribution is broad and that the mechanical strength, such as tensile strength, of the films of the compositions is low.
Accordingly, it is desired to provide an ethylenic copolymer which has a narrow molecular weight distribution and a narrow branching degree distribution, of which the compositional distribution profile can be freely controlled, and which can be formed into films having both high tensile strength and high impact strength irrespective of the molecular weight distribution of the copolymer, and also to provide a composition comprising such an ethylenic copolymer.
Given this situation, the object of the present invention is to provide an ethylenic copolymer which has a narrow molecular weight distribution, a narrow branching degree distribution and a controlled compositional distribution profile, and forms a film having good mechanical characteristics including tear strength, etc., and having good heat-sealing properties and good ESCR; to provide an ethylenic copolymer composition which comprises said ethylenic copolymer and an additional ethylenic copolymer having a narrow molecular weight distribution and a broad compositional distribution and capable of forming films with high tear strength, and which forms a film having both good impact strength and good tear strength, or that is, the tear strength of the film is improved without sacrificing its impact strength; and to provide an ethylenic copolymer film having good characteristics such as those mentioned above.
In order to attain the above-mentioned object, we, the present inventors have assiduously studied and, as a result, have found that an ethylenic copolymer which is obtained through copolymerization of (a) ethylene and (b) at least one selected from xcex1-olefins having from 3 to 20 carbon atoms, diene compounds and cyclic olefins and which has specific properties, and an ethylenic copolymer composition containing said copolymer can attain the intended object. On the basis of these findings, we have completed the present invention.
Specifically, the present invention provides the following:
(1) an ethylenic copolymer (I) obtained through copolymerization of (a) ethylene and (b) at least one selected from xcex1-olefins having from 3 to 20 carbon atoms, diene compounds and cyclic olefins, which is characterized in that (1) the ratio of the weight average molecular weight (Mw) of the copolymer to the number average molecular weight (Mn) thereof, Mw/Mn, falls between 1.5 and 4, said molecular weights being measured through gel permeation chromatography in terms of polyethylene, and the weight average molecular weight (Mw) falls between 3,000 and 1,000,000, (2) the relationship between the half width at the half maximum [W/2] of the curve as obtained through Gaussian distribution approximation relative to the essential peak of the compositional distribution curve of the copolymer obtained through temperature rising elution fractionation analysis, and the average, n, of short-chain branches existing in the copolymer (per 1000 carbons of the copolymer) satisfies the following equation:
0.704+0.147nxe2x89xa6W/2xe2x89xa6xe2x88x920.055+0.577n, 
and (3) the resin density of the copolymer falls between 0.85 and 0.95 g/cm3;
(2) an ethylenic copolymer (II) obtained through copolymerization of (a) ethylene and (b) at least one selected from xcex1-olefins having from 3 to 20 carbon atoms, diene compounds and cyclic olefins, which is characterized in that the ratio of the weight average molecular weight (Mw) of the copolymer to the number average molecular weight (Mn) thereof, Mw/Mn, falls between 1.5 and 4, said molecular weights being measured through gel permeation chromatography in terms of polyethylene, that the resin density of the copolymer falls between 0.85 and 0.95 g/cm3, and that the tear strength (TS, kgf/cm) of the copolymer satisfies the following equations:
TSxe2x89xa7131.5xe2x88x92155xc3x97log[{(B+C+D)/A}+0.1], 
and
0.1xe2x89xa6xe2x88x92(B+C+D)/Axe2x89xa6xe2x88x921 
where A indicates the area of the range which is surrounded by the curve as obtained through Gaussian distribution approximation relative to the essential peak of the compositional distribution curve of the copolymer obtained through temperature rising elution fractionation analysis, and the base line of said compositional distribution curve; B indicates the area of the range which is surrounded by said compositional distribution curve, said curve obtained through Gaussian distribution approximation relative to said essential peak, and said base line, and which is in the lower elution temperature side and is at an elution temperature higher than 32xc2x0 C., while C indicates the area of the range which is surrounded by said three and which is in the higher elution temperature side; and D indicates the area of the range which is surrounded by said compositional distribution curve and said base line and which is at an elution temperature between 25 and 32xc2x0 C.;
(3) an ethylenic copolymer composition (I) comprising;
(A) an ethylenic copolymer which is obtained through copolymerization of (a) ethylene and (b) an xcex1-olefin having from 3 to 20 carbon atoms, and which is such that (1) the ratio of the weight average molecular weight (Mw) of the copolymer to the number average molecular weight (Mn) thereof, Mw/Mn, falls between 1.5 and 4, said molecular weights being measured through gel permeation chromatography in terms of polyethylene, and the weight average molecular weight (Mw) falls between 3,000 and 1,000,000, (2) the relationship between the half width at the half maximum [W/2] of the curve as obtained through Gaussian distribution approximation relative to the essential peak of the compositional distribution curve of the copolymer obtained through temperature rising elution fractionation analysis, and the average, n, of short-chain branches existing in the copolymer (per 1000 carbons of the copolymer) satisfies the following equation:
0.704+0.147nxe2x89xa6W/2xe2x89xa6xe2x88x920.055+0.577n, 
and (3) the resin density of the copolymer falls between 0.85 and 0.95 g/cm3; and
(B) an ethylenic copolymer which is obtained through copolymerization of (a) ethylene and (b) an xcex1-olefin having from 3 to 20 carbon atoms, and which is such that the weight average molecular weight (Mw) of the copolymer as measured through gel permeation chromatography in terms of polyethylene falls between 3,000 and 1,000,000, and that the resin density of the copolymer falls between 0.85 and 0.95 g/cm3;
(4) an ethylenic copolymer composition (II) which is one embodiment of the composition (I) of said (3) and which is characterized in that the ethylenic copolymer of the component (A) in (3) is obtained through copolymerization of (a) ethylene and (b) an xcex1-olefin having from 3 to 20 carbon atoms and is such that the ratio of the weight average molecular weight (Mw) of the copolymer to the number average molecular weight (Mn) thereof, Mw/Mn, falls between 1.5 and 4, said molecular weights being measured through gel permeation chromatography in terms of polyethylene, that the resin density of the copolymer falls between 0.85 and 0.95 g/cm3, and that the tear strength (TS, kgf/cm) of the copolymer satisfies the following equations:
TSxe2x89xa7131.5xe2x88x92155xc3x97log[{(B+C+D)/A}+0.1], 
and
0.1xe2x89xa6(B+C+D)/Axe2x89xa61 
where A indicates the area of the range which is surrounded by the curve as obtained through Gaussian distribution approximation relative to the essential peak of the compositional distribution curve of the copolymer obtained through temperature rising elution fractionation analysis, and the base line of said compositional distribution curve; B indicates the area of the range which is surrounded by said compositional distribution curve, said curve obtained through Gaussian distribution approximation relative to said essential peak, and said base line, and which is in the lower elution temperature side and is at an elution temperature higher than 32xc2x0 C., while C indicates the area of the range which is surrounded by said three and which is in the higher elution temperature side; and D indicates the area of the range which is surrounded by said compositional distribution curve and said base line and which is at an elution temperature between 25 and 32xc2x0 C.; and
(5) an ethylenic copolymer film to be prepared by filming any of said ethylenic copolymer of (1) or (2), or said ethylenic copolymer composition of (3) or (4).